Renal Transplant Ultrasound: Protocol, Doppler, and Rejection Findings

Renal transplant ultrasound is a high-stakes specialty study that you may perform regularly in a transplant center or occasionally as a complex exam in a general lab. The transplanted kidney lives in the iliac fossa (right or left, depending on surgical placement), making it paradoxically easier to image than native kidneys — it's subcutaneous and accessible. But the clinical stakes are high: abnormal findings directly guide decisions about biopsy, intervention, or graft salvage.

Transplant Anatomy

Surgical position: Right iliac fossa (most common) or left iliac fossa. The kidney sits anteriorly, just below the anterior abdominal wall.

Vascular anastomoses:

• Renal artery: Anastomosed end-to-end to the internal iliac artery, or end-to-side to the external iliac artery
• Renal vein: Anastomosed end-to-side to the external iliac vein

Ureter: Anastomosed to the bladder (ureteroneocystostomy). A ureteral stent is often placed initially and removed at 4–6 weeks.

No hilar vessels: Unlike native kidneys, you won't find the artery entering the hilum from the medial side — trace from the iliac vessels to the renal artery origin.

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Why Ultrasound?

Transplant ultrasound is performed:

• Routinely: Immediately post-transplant, then at scheduled intervals (1 day, 1 week, 1 month, 3 months, annually)
• Acutely: Any rise in creatinine, oliguria, fever, graft pain, or hypertension
• Before biopsy: Confirm kidney position and size; identify safe approach
• Follow-up: After any intervention, stent removal, or treatment

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Scanning Protocol

Equipment

Probe: 5–9 MHz curved linear or linear transducer. The kidney is superficial — use higher frequency for better resolution.

Patient position: Supine. The kidney is usually palpable.

Gray Scale Evaluation

1. Kidney size and morphology:

• Length (longitudinal), width, and AP diameter — measure in two planes
• Normal post-transplant kidney: 10–13 cm (similar to native)
• Enlarged, edematous kidney: Suggests acute rejection, ATN, or pyelonephritis

2. Cortical echogenicity:

• Normal: Cortex slightly hypoechoic relative to renal sinus fat
• Increased cortical echogenicity: Rejection, medical renal disease, cyclosporine toxicity
• Loss of corticomedullary differentiation: Rejection (advanced)

3. Renal pyramids:

• Normal: Hypoechoic medullary pyramids, clearly distinct from cortex
• Enlarged, prominent pyramids: ATN (acute tubular necrosis), cyclosporine toxicity — the pyramids appear strikingly large and hypoechoic

4. Collecting system:

• Mild pelvic dilation (< 10–15 mm AP renal pelvis) is normal in the early post-transplant period (mild obstruction from edema, ureteral stent)
• Moderate-to-severe hydronephrosis → obstruction (ureteral stricture, calculus, lymphocele compression) or urine leak

5. Perinephric collections:

• Hematoma: Early post-op — echogenic collection adjacent to kidney
• Urinoma: Urine leak from ureteroneocystostomy or calyx — anechoic collection
• Lymphocele: Most common post-transplant collection; anechoic with thin septations; develops weeks to months post-op; may compress the ureter or iliac vein
• Abscess: Complex collection with internal debris, gas, vascularity

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Doppler Evaluation

Doppler is the most important component of the exam.

Vessels to evaluate:

1. Main renal artery (at the anastomosis and in the hilum)
2. Segmental arteries (3–4 locations in the kidney parenchyma)
3. Interlobar arteries (upper, mid, lower poles)
4. Main renal vein

Resistive Index (RI)

RI = (PSV - EDV) / PSV

Measure at interlobar or arcuate arteries. Take at least 3 measurements in different parts of the kidney and average.

Normal RI: < 0.70 — some labs use < 0.75 depending on patient factors Borderline elevated: 0.70–0.80 Significantly elevated: > 0.80 — concerning for rejection, ATN, obstruction, cyclosporine toxicity

The problem with RI: It is nonspecific. Elevated RI is seen in:

• Acute rejection
• ATN
• Cyclosporine toxicity
• Renal vein thrombosis
• Severe hydronephrosis
• Pyelonephritis
• Any cause of increased interstitial pressure

RI must always be interpreted in clinical context. Biopsy is the definitive test for rejection.

Main Renal Artery — Stenosis Assessment

Transplant renal artery stenosis (TRAS) is the most common vascular complication (2–10%).

Suggestive findings:

• PSV at the anastomosis > 250 cm/s — highly suspicious
• PSV ratio (anastomosis / pre-anastomosis external iliac PSV) > 1.8
• Tardus-parvus waveform in intrarenal arteries: slow upstroke (acceleration time > 0.07 sec), low RI, dampened waveform
• Focal color aliasing or color bruit at the anastomosis

Reporting: If PSV > 250 cm/s, recommend CT angiography or MR angiography for confirmation.

Renal Vein — Thrombosis

Renal vein thrombosis is an early, catastrophic complication (usually within 1 week).

Findings:

• No venous flow in the main renal vein (absent color signal)
• Reversal of diastolic flow in intrarenal arteries — the most characteristic finding; diastolic waveform goes below the baseline
• Absent cortical perfusion on power Doppler in severe cases

If you see reversed diastolic flow in intrarenal arteries on Doppler — call the radiologist immediately. This is a graft-threatening emergency.

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Key Diagnoses and Their Findings

Acute Rejection

• Kidney enlargement
• Cortical echogenicity increased, corticomedullary differentiation reduced or lost
• Elevated RI (usually > 0.80)
• Enlarged medullary pyramids
• No specific Doppler sign — biopsy required to confirm

Acute Tubular Necrosis (ATN)

• Usually in the first days after transplant (ischemia-reperfusion)
• Kidney often enlarged
• Prominently enlarged medullary pyramids — classic finding
• RI may be elevated (0.70–0.90)
• Clinical: creatinine rises immediately post-transplant, then slowly improves

ATN and rejection overlap sonographically — biopsy distinguishes them.

Cyclosporine Toxicity

• Similar to ATN — elevated RI, prominent pyramids
• Clinical context (drug levels) differentiates

Obstruction

• Hydronephrosis (mild in early post-op period may be normal)
• RI may be elevated if longstanding
• Identify cause: ureteral stricture (gradual narrowing on ureteral doppler), lymphocele (compresses ureter), calculus

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Key Images to Capture

1. Kidney in long axis — with and without measurements
2. Kidney in transverse — upper, mid, lower thirds
3. Perinephric space — anterior and posterior to kidney
4. Spectral Doppler: main renal artery (PSV, waveform)
5. Segmental artery RI × 3–4 locations
6. Main renal vein — color and spectral
7. Bladder — for hydroureter or urinoma
8. Any collection — size, contents, location

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Reporting Language

Normal: "Transplant kidney in the right iliac fossa measuring 11.6 × 5.8 × 5.4 cm. Normal cortical echogenicity with preserved corticomedullary differentiation. No hydronephrosis or perinephric collection. Main renal artery waveform demonstrates normal acceleration and PSV 118 cm/s. Mean interlobar RI 0.63 (range 0.60–0.66). Main renal vein: normal phasic flow."

Elevated RI / rejection concern: "Transplant kidney is enlarged at 14.2 cm. Increased cortical echogenicity with reduced corticomedullary differentiation. Mean interlobar RI 0.87 (range 0.82–0.91). No hydronephrosis. No perinephric collection. Elevated RI with parenchymal changes raises concern for acute rejection or other parenchymal dysfunction. Clinical correlation and consideration of biopsy are recommended."

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SonoBuddy's vascular protocols include renal artery stenosis, transplant surveillance, and resistive index calculation guides.